Although the length of the day is known to be a major environmental factor which regulates the timing of the reproductive season in many mammals, the neuroendocrine events which underlie this phenomenon remain unknown. Recent evidence indicates that an alteration in sensitivity of the hypothalamic-hypophyseal axis to testosterone feedback may be one way in which the photoperiod acts to alter pituitary gonadotropin release in seasonally breeding animals. Studies in this proposal are designed to investigate the interaction between the photoperiod and testosterone in more detail, and to determine if the length of the day can alter the sensitivity of the gonadotropin control center to other sex steroids. In view of the apparent similarities between photic-induced changes in steroid-feedback sensitivity and those that occur during sexual maturation, a detailed examination of these processes within the same species will be undertaken. The pineal gland mediates, at least in part, the effect of the photoperiod on hypothalamic-pituitary sensitivity to testosterone, and the role of this organ, and its product melatonin, in this process will be further investigated. We will also determine if the pineal gland is involved in the photic-induced alteration in sensitivity to testosterone which occurs in neural centers controlling sexual behavior. In addition to a steroid dependent process, it appears that the photoperiod can also alter pituitary gonadotropin release independently of gonadal and adrenal steroid hormones. We plan to examine the precise roles of various known components of the photoperiodic system individually (i.e. eyes, suprachiasmatic nuclei, biological clock, pineal gland), in both the steroid dependent and steroid independent steps of the photoperiodic response. It is anticipated that such an approach will enable us to develop a better understanding of the complex sequence of events associated with the photoperiodic control of neuroendocrine-gonadal activity.